While major headlines broke over news that Chicago was the only one of America’s largest 20 cities to shrink from July 2015 to July 2016, most of Illinois’ other cities with 50,000 people or more also lost population.

If an Illinois worker takes a pay cut during a recession, she knows the state isn’t going to take an even bigger chunk out of her paycheck. That’s because the state income tax rate stays the same. But if her home loses value, too, she could still see her property tax bill go up. Government...